In Act 2, Scenes 4-6, of Romeo and Juliet, how is Mercutio both the jester and friend?

Mercutio only appears in Act II scene 4, but he is
definitely seen in this scene as playing the dual role of jester and friend to Romeo.
Note the way in which he engages in a battle of wits with Romeo, showing his
intelligence but also his humour through elaborate punning such as the following example
shows:

That's as much as to say, such a case as yours
constrains a man to bow in the hams.

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Here, Mercution is playing on the alternative
meaning of "case" which is a set of clothes, and "hams" to mean
legs.

However, at the same
time, this scene starts off with Mercutio being concerned for his friend Romeo,
wondering where he is and what has happened to him:

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Where the devil should this Romeo
be?

Came he not home
tonight?

Thus we can see in
this scene that Mercutio performs the dual function of friend and jester, both roles
which are accomplished by him brilliantly.